The number of devices using some form of wireless communication to communicate is rapidly increasing in multiple application areas, such as in lighting control, building or city management, high-speed Internet access, security and surveillance, home automation and industrial monitoring and control systems. As a result of the wide-spread adoption of wireless networking technology, the interference between different systems is increasingly becoming a factor that affects the operational performance of such networked systems. This is especially true since many systems will use unlicensed industrial, scientific and medical (ISM) bands, like in the sub-1-GHz band and in the 2.4 GHz ISM band. For instance, in the latter band there are multiple systems for different application scenarios, e.g. WiFi, Bluetooth, and Zigbee.
A system used for medical or lighting monitoring and control applications, should be robust and, consequently, include an interference agility mechanism, which can deal with certain levels of interference. However, the performance of systems using wireless radio communication can be degraded or obstructed in the presence of radio interference. Interference can be caused by any electrical device that emits radio waves with sufficient power and uses a frequency that is approximately equal to the operating frequency used by the affected system.
As an increasing number of devices are sharing the same bands, the issue of coping with radio interference is becoming more important than ever before. Radio interference has the potential to severely degrade the network performance of any system that is communicating wirelessly. This makes it essential to develop techniques that can help mitigate the problem of interference.
In a multi-channel system (e.g. Zigbee and other 802.15.4-based systems having 16 channels in the 2.4 GHz ISM band, and 4 to 10 bands in the sub-GHz band), a possible solution to solve the interference issue is to move the system's working frequency to a clean channel. This is relatively simple in single hop network applications, for example, a wireless mouse. A simple synchronization between peer devices is sufficient to ensure successful channel switching.
In a multi-hop mesh topology, all nodes serve as potential relay (intermediary) nodes of an end-to-end routing function. As a result of the redundancy in routes between any two nodes and the self-organizing nature, mesh topologies are robust to changes in the ambient environment. However, they also increases the complexity of switching the networked system to a different carrier frequency.
The easiest approach would be to introduce a new system/network management message specifying the new channel. However, some nodes may not receive this message (or not in time) due to message loss (e.g. due to interference), temporary unavailability of nodes (e.g. since they are not switched to the power), message delays, etc. This is an important issue in a mesh network because nodes that did not receive the “channel switch” message, will not change to the indicated channel at the specified time, possibly resulting in a disconnected network when the affected node(s) is/are on a critical routing path.
How does the system efficiently and effectively deals with nodes that did not switch to the new channel (subsequently referred to as “orphan nodes”)? How does the system synchronize all nodes (especially the nodes who act as routers for messages) to move to the new channel together? How can the system guarantee that the whole system moved to the new channel? It is clear that many issues are to be solved in the mesh network context, concerning channel switching. Otherwise, the system cannot work properly and will not provide a sufficient quality of service.